The heat pump will start, or try to start, the furnace if the difference between the setting and actual inside temperature is greater than about 6 degrees. If you set the thermostat to 70 and it's 63 inside, the furnace is used until the inside temperature comes up.
Our rear furnace originally had a problem with its thermostat. A small wire had broken in the thermostat. It could be something that simple.
We recently had a problem with our front furnace. To spread the heat from the rear furnace, I turned off the front thermostat and set the fan to "on" instead of "auto". A day or two later, the front furnace decided to start working again. I blame gremlins